Gas turbines, used in power plants for example, typically have multiple combustion chambers. The combustion chambers are termed “cans” in the art. The cans have variation in fuel flow and air flow due to variation in an associated fuel and air distribution system. Consequently, this variation manifests itself in terms of fuel to air ratio variation, which leads to variation in temperature, dynamics (pressure vibration) and emissions across the combustion chambers or cans. The can to can variation or stratification also contributes to turbine exhaust temperature variation. Another important factor that contributes to exhaust temperature variation is variation in circumferential and axial expansion (that determines temperature and pressure gradients) over the turbine stages due to flow variation and geometry.
The can to can variation in terms of fuel to air ratio leads to some cans being hotter, i.e. higher flame (or firing) temperature than others due to higher fuel to air ratio than other cans. These cans exhibit higher Nitrogen Oxides (NOx) emissions and certain pressure dynamic spectral tones (to be defined later in this patent) corresponding to higher flame temperature tend to be stronger. On the other hand, this variation can lead to one can burning very lean or almost “blowing out” (i.e., flame extinguishes), if for example, the fuel to air ratio is below a certain threshold The blowout of a combustion chamber or a can is termed “Lean Blow out” or LBO. Colder cans have higher LBO risk and higher Carbon Monoxide (CO) emissions due to leaner fuel to air ratio than hotter cans that have higher NOx emissions due to higher fuel to air ratio. Colder cans also have certain dynamic tones that respond to colder firing temperature, i.e., tones that increase in amplitude as firing temperature decreases. If it were possible to monitor firing temperature of each can, it would help to balance the cans by changing fuel or airflow to the can. However, due to the extreme temperatures and operating conditions within the cans, temperatures sensors cannot be currently located in each can to monitor the temperatures within each can as the present temperature sensing technology cannot withstand such harsh conditions. Instead, in the art, pressure dynamics are measured for combustion chambers or cans and are used as an indicator of “hotness” or “coldness” of a can. There are certain dynamic tones (as will be explained later) that can be used to estimate the firing temperature of the can. Using pressure vibration sensors, feedback for each can, fuel flow and airflow is scheduled at the global or turbine level (total air and fuel for all the cans) to meet turbine load requirements such that the combustion dynamics in each can and emissions at the turbine level are within acceptable limits. If emissions be measured at the can level, then the objective would be to achieve emissions compliance at the can level. Specifically, according to current combustion tuning practice, the overall fuel splits from the fuel system to the cans and the bulk fuel flow are set through the main fuel gas control valves.
Tuning of a multiple-chamber combustion system is driven by the following constraints: 1) maintaining the gas turbine unit emissions below a set target across a pre-defined load range and 2) maintaining the individual can combustor dynamics below acceptable limits across the load range. Accordingly, the tuning process attempts to set the configuration of the main gas control valves such that the worst can has combustor dynamics below an acceptable limit. In this process, the overall operability window is set by the combustion response of either the “richest” (highest fuel to air ratio (f/a)) can or the “leanest” (lowest fuel to air ratio (f/a)) can. The variation in the response of the individual combustion chambers is hereafter referred to as “can-to-can” variation. In order to address this can level variation, trim devices such as but not limited to valves, orifice plates, etc. that can control flow to individual cans are needed. This helps increase the operability window by making all the cans fire uniformly. This ensures uniform degradation of hardware making maintenance easy. Any reduction in can to can variation provides an uprate opportunity in terms of firing temperature and hence power output subject to hardware (temperature limits) and emissions constraints. This in other words implies more output with acceptable emissions.
Additionally, exhaust gas temperatures have been examined in methods like that shown in U.S. Patent Application US 2002/01 83916 A1 to identify malfunctioning combustion chambers. In said application, is noted that typically in the art, a turbine must be shut down and examined to determine which cans are malfunctioning. Therefore, to avoid this loss of time and expense, a system that can monitor the cans while the turbine is operating is desirable so as to enable online tuning of fuel to air (f/a) ratio of the cans to reduce can to can variation in terms of dynamics, reduce emissions and provide an opportunity of increased output subject to emissions and hardware life constraints.
Thus, a method for determining and dealing with can-to-can variations and addressing it by tuning f/a ratio is needed to ensure uniform life of the cans and to provide more efficient operation of the turbine with opportunity for increased output and reduced emissions.